Variables Motivation This series introduces the concept of - - PowerPoint PPT Presentation

Variables

Module 2

Motivation

• This series introduces the concept of variables

§ Very powerful programming concept § Necessary for more complex Python features

• But variables can be tricky to work with

§ With expressions, we got a value right away § A lot of variable features happen invisibly

• This can lead to lot of frustration

§ You think Python is doing one thing § It is actually doing something else

Visualization

• You need to learn to think like Python thinks

§ Otherwise you and Python will miscommunicate § Like a coworker with language/cultural issues § A good programmer sees from Python’s persp.

• Do this by building visual models of Python

§ You imagine what Python is doing invisibly § Not exactly accurate; more like metaphores § We call this skill visualization § It is a major theme of this course

Variables

• A variable

§ is a box (memory location) § with a name § and a value in the box

• Examples:

5 x

Variable x, with value 5 (of type int)

20.1 area

Variable area, w/ value 20.1 (of type float)

1.5

x

Variables in Python

• These boxes represent a “memory location”
• Allows variables to be used in expressions

§ Evaluate to the value that is in the box § Example: 1 + x evaluates to 6

• Allows variables to change values

§ Example: § They can even change the type of their value § This is different than other languages (e.g. Java)

5

x

5

x

1.5

x

Creating Variables

• So how do we make a variable in Python?

§ Cannot do just with expressions § Expressions give us a value § We want to command Python to make a box

• Variables are created by assignment statements

x = 5

• This is a statement, not an expression

§ Expression: Something Python turns into a value § Statement: Command for Python to do something x the value the variable 5

Expressions vs Statments

But can now use x as an expression Value NOTHING!

Naming Variables

• Python limits what names you can use

§ Names must only contain letters, numbers, _ § They cannot start with a number § Also cannot be a reserved word (will see later)

• Examples

§ e1 is a valid name § 1e2 is not valid (it is a float) § a_b is a valid name § a+b is not valid (it is an + on two variables)

Variables Do Not Exist Until Made

• Example:

>>> y Error! >>> y = 3 >>> y 3

• Changes our model of Python

§ Before we just typed in one line at a time § Now program is a sequence of lines

Variables Do Not Exist Until Made

• Example:

>>> y Error! >>> y = 3 >>> y 3

• Changes our model of Python

§ Before we just typed in one line at a time § Now program is a sequence of lines >>> x = 3 >>> y = 4 >>> x+y 7 >>> x = 2.5 >>> x+y 6.5

Assignments May Contain Expressions

• Example: x = 1 + 2

§ Left of equals must always be variable: 1 + 2 = x § Read assignment statements right-to-left! § Evaluate the expression on the right § Store the result in the variable on the left

• We can include variables in this expression

§ Example: x = y+2 § Example: x = x+2

This is not circular! Read right-to-left. x 5 y 2

Assignments May Contain Expressions

• Example: x = 1 + 2

§ Left of equals must always be variable: 1 + 2 = x § Read assignment statements right-to-left! § Evaluate the expression on the right § Store the result in the variable on the left

• We can include variables in this expression

§ Example: x = y+2 § Example: x = x+2

This is not circular! Read right-to-left. x 5 y 2

x 4

Assignments May Contain Expressions

• Example: x = 1 + 2

§ Left of equals must always be variable: 1 + 2 = x § Read assignment statements right-to-left! § Evaluate the expression on the right § Store the result in the variable on the left

• We can include variables in this expression

§ Example: x = y+2 § Example: x = x+2

This is not circular! Read right-to-left. x 5 y 2

x 4 6 x

About Crossing Off

• The crossing off is a helpful mental model

§ Emphasizes that the old value was deleted § But it shouldn’t stay there over time § Else might think a box remembers old values

• So what do we do?

§ Cross off at the time we execute the statement § But gone when we revisit the variable later

• Again, part of our visualization

Python is Dynamically Typed

• What does it mean to be dynamically typed?

§ Variables can hold values of any type § Variables can hold different types at different times

• The following is acceptable in Python:

>>> x = 1 >>> x = x / 2.0

• Alternative is a statically typed language

§ Each variable restricted to values of just one type § Examples: Java, C, C++ ç x contains an int value ç x now contains a float value

Dynamic Typing

• Often want to track the type in a variable

§ Would typing x+y cause an error? § Depends on whether x, y are int, float, or str values

• Use expression type(<expression>) to get type

§ type(2) evaluates to <class 'int'> § type(x) evaluates to type of contents of x

• What is a class?

§ In Python it is a synonym for a type § In Python 2, the word type would be there

Going Meta

• The types are themselves values!

§ Can assign them to variables: x = int § Can use them in expressions: int == float

• What is their type? It is type.

§ type(int) evaluates to <class 'type’>

• Can use in a boolean expression to test type

§ type('abc') == str evaluates to True § type(x) == str evaluates to False